This is a very common question asked in discrimination complaints. Your witnesses may still be employed by the same employer you are suing.
The employer may have made it clear that it will retaliate against anyone who takes your side, or this threat may just be implied. While an agency may provide some protection for these witnesses, if the case goes to trial, then they are not protected.
You cannot blame a witness for not wanting to put his or her job on the line, especially in today’s economy.